The most common obstacle used to prevent passage by persons, animals or machines is a fence. Different fences are used for different purposes. When denying passage to humans, the conventional form of fence is a high chain-link fence. When denying passage is particularly important, barbed wire is added to the top of the fence. In cases of more important denied passage, the barbed wire may be replaced by razor wire. Loosely coiled concertina wire may be used in place of stretched strands to further create a bar to human passage. When it is extremely important that passage be denied, two or more fences are imposed. As an additional obstacle, the fences may be electrified.
Fences as a means of preventing human passage have one outstanding common feature. The fences are ugly and disfiguring of the landscape.
The disfigurement and ugliness of fences is particularly important when considering psychological impact on people living outside the fences, as well as people living inside the fences.
Social factors and changes in patterns of crime require more prisons. There is little space to devote to prisons. There may be reluctance to build prisons in view of the ugly nature of the necessary fences, as well as the ineffectiveness of conventional fences.
Military bases require anti-personnel fences. The existence of some particular type of fences often unintentionally reveals the secrecy or importance or nature of the location.
Conventional fences have one common disadvantage, which is the susceptibility of the fences to defeat by wire cutters. After cutting wires of a fence, one simply walks through where the fences have been, walking beneath the stranded or concertina form of anti-personnel wire on top of the fences and effectively defeating much of the added material of the fences.
Another common problem is that fences may be scaled. Throwing a coat or canvas across the barbed or razor wire at the top, one simply climbs a fence and drops down on the other side.
A need exists for better fences to surround prisons and military fortifications and the like.